Most new wireless communications devices now include either embedded Global Positioning System (GPS) receivers or have suitable ports/interfaces for connecting to external GPS units, such as Bluetooth™-enabled GPS pucks. These GPS-enabled wireless communications devices can thus enable a variety of location-based services (LBS), such as turn-based navigation.
When a user wishes to start using GPS-based navigation or other location-based software on a wireless handheld device equipped with a GPS receiver, a precise location fix must first be established. This is done by synchronizing with at least four GPS satellite vehicles (SV's) in orbit. The time required to compute the location on a new synchronization is known as time-to-first-fix (TTFF). Unfortunately, this interval can become impractically long for various reasons. In fact, it can take 20 minutes or significantly longer to acquire a location fix in poor conditions, rendering navigation software unusable until then. As a result, the user can easily become frustrated.
In order to be able to find the satellites initially, a device with a GPS receiver normally contains an almanac of satellite location data that is imprecise but valid for several months. If a full reset occurs, then downloading the full almanac takes 12.5 minutes, with a maximum wait of 25 minutes. The almanac serves as a rough approximation of satellite location. To acquire a precise fix, the device must also obtain ephemeris data from each satellite itself which consists of very precise orbital and clock correction data, at a slow 50 bytes per second for a total of 12 seconds for ephemeris and 6 seconds for clock corrections. When the device retrieves this data directly from a satellite broadcast, it is operating in autonomous or standalone GPS mode, the most common mode of operation. The ephemeris data is cached by the receiver, but it becomes stale and unusable within 3 to 4 hours due to satellite drift due to various sources of error including ionospheric effects (which introduces an error of ±5 meters), ephemeris errors (which introduce an error of ±2.5 meters), satellite clock errors (±2 meters), multipath distortion (±1 meter), tropospheric effects (±0.5 meters), numerical errors (±1 meter or less). If the wireless handheld device loses synchronization because, for example, the user enters a building or subway and the ephemeris becomes stale, then a “cold start” condition occurs and new ephemeris must be downloaded again through a full sky search.
A problem that arises in addition to the latency involved in the broadcast data download is that one or more obstructions in the signal path can further delay the TTFF. Satellites broadcast their ephemeris every 30 seconds, 5 times per window. If the signal is interrupted, the receiver must wait for another cycle. Obtaining a fix can easily take several minutes or much longer if the user is travelling through an urban canyon or dense foliage. On a hot start, where current ephemeris data is still available, the time required to obtain a fix is reduced to seconds. However, if a user turns on the handheld device or emerges from a building so that the receiver has no valid ephemeris data in memory, then the user may typically need to wait a few minutes before a fix is established.
Also, the geography may be such that insufficient SV's are ever visible to obtain a fix through autonomous mode, as a reasonably high signal level of −130 dBm is required to download ephemeris. Thus, the user may be unable to reliably acquire a fix indefinitely.
To obtain a rapid TTFF, short-term ephemeris data can be obtained for Assisted-GPS (A-GPS) on networks such as CDMA. This consists of a system where the handheld GPS device obtains assistance data over-the-air from the nearest cellular network base station. Once the data is received, the handheld device combines this assistance data with ranging information on the satellites it can see or, alternatively, for a more precise fix, performs satellite pseudo-range calculations that are sent to the network's location server, which then returns a more precise location estimate. The device can then acquire and track weaker satellite signals down to −155 dBm in this mode of operation. Unfortunately, Assisted GPS has not been widely deployed in North America and Europe. The user may need to pay a charge per fix or a flat monthly fee to use the service, and the data is only good for about 4 hours.
An increasingly popular solution is to implement Extended Ephemeris (EE), also known as Aided-GPS. This new technology involves retrieval of current raw ephemeris from satellite broadcasts recorded by a reference network of receivers positioned strategically around the globe. Modelling is applied to the data to predict the future path of satellites, and so ephemeris is calculated for the next few days or longer. This extended ephemeris data is then made available for distribution from redundant location servers that can be accessed by mobile devices through TCP/IP on any wireless network. The data must initially be downloaded through a carrier network, but it does not rely on any specific implementation of GPS assistance by the carrier, making it useful around the world. The extended ephemeris can be used to reduce the TTFF to a couple of seconds even in the absence of network connectivity.
Despite these advances, the manner in which ephemeris data is downloaded, be it for Assisted GPS or for Aided GPS, is not optimal. Extended ephemeris data for Aided GPS is obtained by wireless communications devices at regular, predetermined intervals (e.g. every 3 days) without regard to the usage patterns of the user. For Assisted GPS, the ephemeris data is only requested when required, and thus conventional methods of obtaining ephemeris for Assisted GPS technology do not anticipate when ephemeris data is likely to be required. In both the Assisted and Aided GPS scenarios, the conventional manner of obtaining ephemeris data is “unintelligent” because it does not take into account the usage patterns of the user of the device. Accordingly, a method for intelligently obtaining ephemeris data for a wireless communications device remains highly desirable.
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.